The present invention relates generally to integrated circuits and, more particularly, to an antifuse programming circuit with a snapback select transistor.
Integrated circuits are interconnected networks of electrical components fabricated on a common foundation called a substrate. The electrical components are typically fabricated on a wafer of semiconductor material that serves as a substrate. Various fabrication techniques, such as layering, doping, masking, and etching, are used to build millions of resistors, transistors, and other electrical components on the wafer. The components are then wired together, or interconnected, to define a specific electrical circuit, such as a processor or a memory device.
Fusible elements are employed in integrated circuits to permit changes in the configuration of the integrated circuits after fabrication. For example, fusible elements may be used to replace defective circuits with redundant circuits. Memory devices are typically fabricated with redundant memory cells. The redundant memory cells may be enabled with fusible elements after fabrication to replace defective memory cells found during a test of the fabricated memory device. Fusible elements may also be used to customize the configuration of a generic integrated circuit after it is fabricated, or to identify an integrated circuit.
One type of fusible element is a polysilicon fuse. The polysilicon fuse comprises a polysilicon conductor fabricated to conduct electrical current on an integrated circuit. A portion of the polysilicon fuse may be evaporated or opened by a laser beam to create an open circuit between terminals of the polysilicon fuse. The laser beam may be used to open selected polysilicon fuses in an integrated circuit to change its configuration. The use of polysilicon fuses is attended by several disadvantages. Polysilicon fuses must be spaced apart from each other in an integrated circuit such that when one of them is being opened by a laser beam the other polysilicon fuses are not damaged. A bank of polysilicon fuses therefore occupies a substantial area of an integrated circuit. In addition, polysilicon fuses cannot be opened once an integrated circuit is placed in an integrated circuit package, or is encapsulated in any manner.
Another type of fusible element is an antifuse. An antifuse includes two conductive terminals separated by an insulator or a dielectric, and is fabricated as an open circuit. The antifuse is programmed by applying a high voltage across its terminals to rupture the insulator and form an electrical path between the terminals. Another type of antifuse may be implemented using a transistor. Under high voltage, the gate dielectric layer ruptures, causing a short to substrate. In either case, the electrical path created by programming the antifuse can later be detected and used as the basis for configuring the device.
Antifuses have several advantages that are not available with typical fuses. A bank of antifuses takes up much less area of an integrated circuit because they are programmed by a voltage difference that can be supplied on wires connected to the terminals of each of the antifuses. The antifuses may be placed close together in the bank, and adjacent antifuses are typically not at risk when one is being programmed. Antifuses may also be programmed after an integrated circuit is placed in an integrated circuit package, or encapsulated, by applying appropriate signals to pins of the package. This is a significant advantage over polysilicon fuses for several reasons. An integrated circuit may be tested after it is in a package, and may then be repaired by replacing defective circuits with redundant circuits by programming selected antifuses. A generic integrated circuit may be tested and placed in a package before it is configured to meet the specifications of a customer. This reduces the delay between a customer order and shipment. The use of antifuses to customize generic integrated circuits also improves the production yield for integrated circuits, because the same generic integrated circuit may be produced to meet the needs of a wide variety of customers.
An exemplary antifuse programming circuit 100 is shown in FIG. 1. The antifuse programming circuit includes a programming terminal 105 to which an external programming voltage is applied for programming an antifuse 110. The antifuse is coupled to an isolation transistor 115 and a select transistor 120. The isolation transistor 115 provides voltage isolation due to the relatively high programming voltage required to program the antifuse 110 thereby protecting other circuit elements from damage (e.g., the select transistor 120). The isolation transistor 115 only passes its gate voltage minus a threshold voltage to its source.
The application of the program voltage ruptures the dielectric of the antifuse 110, creating a conductive path through the transistors 115, 120. After the initial rupture, the program voltage is applied for a specified time interval to allow current to flow through the antifuse 110 thereby reducing the resistance of the conductive path through the antifuse 110. This specified time interval is commonly referred to as a soak interval. The level of soak current required to program the antifuse 110 and provide a reliable restive path is typically significant.
Although antifuses are typically more compact than other types of fusible elements, such as polysilicon fuses, they still consume an appreciable amount of real estate on the semiconductor device. With reference to FIG. 1, because the transistors 115, 120 must have sufficient current ratings to conduct the soak current, they are typically relatively large transistors. When the relatively large size is accumulated over the number of transistors 115, 120 needed to support a bank of antifuses, the amount of real estate consumed is significant.
This section is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.